In general, a rear-wheel drive vehicle is configured such that a driving force generated by a front engine is transmitted to the rear wheel through a drive shaft connected to a transmission, so that the vehicle moves forward. The engine and transmission are mounted on the front of the vehicle. However, the vehicle is driven by driving the rear wheel instead of the front wheel of the vehicle. In the rear-wheel drive vehicle, the rear wheel is driven and the front wheel is steered. Therefore the rear wheel drive vehicle has comparative advantage in steering performance than the front wheel type. In details, the front and rear momentum are in balance for which it produces perfect balance.
FIG. 1 shows a power transmission structure of the rear-wheel drive vehicle based on a conventional technology. As shown in FIG. 1, since the engine and transmission are located in the front wheel 1, a drive shaft 10 is installed between the transmission and a rear axle and transmits a driving force. Connection portions are connected respectively by means of a universal joint. The drive shaft 10 is generally, as shown in FIG. 1, composed of two metal tubes 11 and 12. The drive shaft 10 is divided into two parts because the length of the drive shaft 10 is very large. It is better that the drive shaft 10 is manufactured with one part. However, the drive shaft 10 is divided into two parts because of a resonance frequency due to the vibration caused by driving a vehicle. In other words, if the drive shaft 10 is manufactured with one part, it is highly probable that a natural frequency and resonance are generated due to the vibration caused by driving a vehicle and it is not safe. Metal yokes 13, 14 and 15 for connection are installed at both ends of each of the two front and rear metal tubes 11 and 12 in the drive shaft 10. Particularly, since the two metal tubes 11 and 12 are connected to each other in the middle of the drive shaft 10 by means of the yoke 14, a center support bearing 16 is provided and fixed to the vehicle chassis in order to maintain the straightness.
Meanwhile, a conventional technology is disclosed in the publication of Korean Patent Application No. 10-2006-0053299. This invention relates to a connection structure between the propeller shaft and the rear axle shaft. The structure of the drive shaft designated as the propeller shaft is well shown. As mentioned above, there are still problems for the conventional technology, since the connection structure is composed of two metal tubes.
The problem caused by the drive shaft is composed of two tubes is at present a serious obstacle to the technology development for high fuel efficiency and being lightweight.
Meanwhile, the friction-stir welding has been developed by British TWI (The Welding Institute) in 1991. The friction-stir welding is a new welding process in which materials are melted into a semi-solid state by the heat generated by putting and rotating a friction tool, and then welding is performed with the mixture of the materials by the rotation of the friction tool. The friction-stir welding has fewer problems such as a protruding portion, periphery transformation due to high temperature heat, and residual stress, etc., which a general welding has. The friction-stir welding was used in aluminum welding in the early days, and recently is used in a variety of materials through technology developments. It can be also used in a liquid hydrogen fuel tank of a spaceship. However, it is difficult to apply the friction-stir welding to a complicated structure.
The friction-stir welding has the above advantages. However, the friction-stir welding is still a kind of welding. Therefore, since the friction tool moves on the bonding interface, a divot, i.e., a concave portion, is formed on that portion. Problems like residual stress, fatigue concentration, etc., which are less than those of the conventional technology occur on the bonding interface.
To solve the problems, a conventional technology is disclosed in the Korean Patent No. 0445142. This conventional technology discloses a friction-stir welding technology capable of preventing a concave portion which extends below the bonding surface from being formed in the bonding portion.
In the conventional technology, a thick portion protruding toward a rotating body bonding mechanism is provided at the end of the bonding portion of a frame member to be bonded. When two thick members, i.e., adjacent members to be bonded come in contact with each other, a trapezoidal shape is obtained. The rotating body bonding mechanism includes a front end having a small diameter and a portion having a large diameter. The rotating body bonding mechanism is inserted into the thick portion. In a position where the front end having a small diameter is first inserted and the portion having a large diameter is overlapped with the thick portion and does not extend below the top surface of the a non-thick side of the member to be bonded, the rotating body rotates and moves along the bonding portion. Despite a gap between the thick portions, a desirable bonding can be performed. After bonding, the remaining portion other than the thick portion is cut by a machine, and thus, a smooth surface is formed. According to the conventional technology, through the method as described above, the divot is prevented from being formed on the bonding interface.
However, in the use of the above technology, the portions of the adjacent members to be bonded, which first come in contact with each other, should be manufactured to form a trapezoid. This requires a increased cost. A shape which is not easy to manufacture requires a greater cost.